This invention relates to electrical connectors for making an electrical connection between a conductor and the conductive shield of a communication cable.
Communication cables are constructed with a core of conductors surrounded by a protective sheath. The sheath is usually comprised of a tubular metallic shield with an outer tubular plastic jacket. Most commonly the shield is aluminum, has a thin insulating coating and is bonded to the outer jacket.
In a cable system, shield continuity and ground must be maintained for optimum performance. Thus at various points in a cable system, for example at a splice, a bonding conductor must be connected to a cable shield to provide this continuity and ground.
Because the shield is usually not separable from the jacket, a shield connector must be attached to the combined shield and jacket forming a layered structure. While a typical shield connector may make good electrical contact initially, flow of the plastic jacket can sufficiently reduce contact force between connector and shield to cause a bond failure after a few years. Efforts have been directed to improving contact force by substituting for the usual nut and bolt connector joining means a sleevelike fitting provided with annular grooves adapted to interfit with protuberances on a terminus of a connecting post. See U.S. Pat. No. 3,643,006. However in this device, as in others, for example those of U.S. Pat. Nos. 3,753,204, 3,753,213 and 4,026,619, the eventual permanent deformation of the plastic jacket with age will lead to reduction of the contact force and possibly even to loosening of the connector's mechanical bond to the cable sheath.
Some shield connectors have spring action to compensate for these changes, thereby minimizing the contact force reduction. These connectors generally fall into two categories:
(1) The cantilever types have pivoting top and bottom plates encompassing a portion of the sheath which are pulled together by a joining means external to the contact area, between the pivot and the contact area. Examples are the devices of U.S. Pat. Nos. 3,778,749 and 3,787,797. PA1 (2) The most common types are those with a centrally located joining means pulling top and bottom plates together in the contact area. In this case the joining means passes through a hole or slit in the sheath. Examples are the devices of U.S. Pat. Nos. 3,676,836 and 3,701,839.
The first type of connector has the advantage of a potentially large "travel" in spring action. Its primary disadvantage is low initial contact force, typically one-half the tension in the joining means. The second type provides contact force approximately equal to the tension in the joining means, but it has potentially small "travel" in the spring action and therefore does not compensate for jacket plastic flow very well.
In view of these problems with existing shield connectors, design criteria for an improved shield connector might include high initial contact force together with a long "travel" spring action to compensate for jacket plastic flow.
Fault currents and lightning surge currents also have been known to cause shield bond failures or damage to the cable core conductors, so a shield bond should be highly resistant to these damaging currents. In particular the resistance of the shield bond must remain low when subjected to such currents.
Shield bond failure can also result from damage to the shield during connector installation. This can occur due to bending and weakening of the shield or simply by cutting the shield in a manner that reduces its current carrying capacity. A shield connector should not damage the cable shield during installation.
A mechanically firm bond is also an important requirement since any tendency to looseness or relative motion in the bond components can aggravate the previously mentioned failure modes.